Why does Rendering Reflection + Refraction take much longer than Rendering them Separately?
Enabling both reflection and refraction enables new light paths that are not rendered when only one of them is enabled. Instead of just reflection + reflection or refraction + refraction it now also has to render reflection + refraction and refraction + reflection.

Further, at each depth Blender will trace both a reflection and a refraction ray, which can make the number of rays to trace go up quickly. For ray depth 1, 2, 3, 4, .. you get 2, 4, 8, 16, .. rays.

Cycles on the other hand will randomly pick between tracing either a reflection or a refraction ray on each bounce. This avoids the quick increase in number of rays for a single sample, but it also means each individual sample contains more noise.